The present invention relates to an arrangement for a polychrome display, comprising a light source and linear polarisation filters.
It has already been proposed in German patent application No. OS 2,148,378 (published Apr. 5, 1973) to provide a nematic liquid crystal cell between crossed polarising films of which one is a selective polariser. Only a specified band of wave lengths in the visible spectrum of unpolarised light passing through the selective polariser is linearly polarised. In the "field-off" state of the nematic liquid crystal cell the optic axis of the cell is uniformly normal to the incident light and the state of polarisation of the light is not therefore changed. The light passing through the analyser will therefore have the color of the transmitted band which traversed the first polariser without having been selectively polarised. In the "field-on" state the optical axis of the nematic layer ceases to be parallel to the direction of the incident light and the nematic layer therefore considerably affects the original state of polarisation of the incident light. Consequently about half the light that had originally been polarised at 90.degree. in relation to the analyser will now pass and complement the unpolarised portion of the spectrum to form a nearly white light.
In a second arrangement likewise described in the above-mentioned German patent application use is made of the depolarisation of light induced by an electric field in a nematic liquid crystal film. In this arrangement a nematic layer is interposed between two crossed linear polarising filters and a passive birefringent optical wave plate or retardation plate is placed between the liquid crystal film and the analyser. In the "field-on" state the optical axis of the liquid crystal layer is uniformly parallel to the incident light so that white light passing through the layer remains linearly polarised until it falls on the retardation plate. The light transmitted by the analyser is colored because of the selective interference between components of the ordinary and the extraordinary ray separated by the birefringence of the plate. The color which actually appears is determined by the thickness and the birefringence of the plate. In the "field-on" state the nematic layer depolarises the light and the light traversing the retardation plate will also be depolarised. In the "field-on" state the analyser will therefore transmit white light.
It is a shortcoming of both these arrangements that substantially they control only one color. Two colors can be controlled by the addition of a conventional color filter of the absorption type for coloring the white light traversing the arrangement when the "field is on", but this color must also "contain" the color which is transmitted by the arrangement when the "field is off". It would therefore be impossible to control two pure primary colors by this method. In the first of the two Siemens arrangements the color contrast between the "field-on" and the "field-off" states is limited by the working principle of the described method and not by the selective polariser itself. In the "field-on" state the transmitted intensity of the selectively unpolarised spectral band is still greater than that of the remainder of the visible spectrum. The generation of several colors by a serial combination of two or more such assemblies comprising polarisers which absorb selectively in different spectral wave bands would not therefore be very effective. In the second above-described arrangement three primary colors could be controlled by serially associating three assemblies, each containing a retardation plate suitable for one primary color. This technique would, however, also be rather inefficient because in the "field-on" state each assembly would transmit less than half the incident light.